Foamed resinous structures are useful in a wide variety of applications such as thermal insulation, in cushions, as packaging, and as adsorbents. Extruded foams are generally made by melting a polymer together with any desired additives to create a polymer melt. A blowing agent is mixed with the polymer melt at an appropriate temperature and pressure to produce a foamable gel mixture. The foamable gel mixture is then cooled and extruded into a zone of reduced pressure, which results in a foaming of the gel and the formation of the desired extruded foam product. As will be appreciated, the relative quantities of the polymer(s), blowing agent(s), and additives, as well as the temperature and manner in which the pressure is reduced will tend to affect the qualities and properties of the resulting foam product.
Traditional blowing agents used for extruded foam products include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). One of the advantages of both CFC and HCFC blowing agents is their high solubility in a polymer melt during the manufacturing process. Higher blowing agent solubility promotes a reduction in viscosity when the blowing agent is mixed with the polymer melt. In turn, lower viscosity leads to lower energy requirements for mixing. On the other hand, a major disadvantage to these traditional blowing agents is that an increasing number of governments worldwide have mandated the elimination of CFC and HCFC blowing agents due to growing environmental concerns. CFCs, and many other halocarbons, have come to be recognized as serious global environmental threats due to their ability to cause stratospheric ozone depletion and global warming. The ozone depletion and global warming impact of chemicals such as CFCs and HCFCs are measured by the ozone depletion potential (ODP) and global warming potential (GWP) respectively.
In view of the mandatory phase out of blowing agents with a high ODP and a high GWP, there has been a movement to replace the conventional blowing agents with more environmentally friendly blowing agents, such as hydrofluorocarbons (HFCs) and CO2, in insulating foam applications. Although HCFCs provide a superior thermal barrier compared to HFC and CO2, the chlorine present in the HCFCs possesses an ozone depletion potential. Additionally, over time, the chlorofluorocarbon gas phase remaining in the foam is released into the atmosphere, thereby reducing the insulative value of the foam and potentially further contributing to the global warming potential. In addition, each of the “non-conventional” blowing agents leads to a different cell size and morphology, depending on the particular blowing agent chosen. Additionally, the cell sizes of the foams produced by these generally environmentally friendly blowing agents are too small to provide an acceptable insulative value to the foamed product and generally results in a higher density and a more costly product. For instance, HFC-134a is much less soluble in a polystyrene melt than HCFC-142b. A, HFC-134a produces foams with a small cell size, which creates difficulty in processing compared to HCFC-142b.
Attempts have been made in the art to enlarge the cell size while utilizing a non-HCFC blowing agent and maintaining an environmentally friendly product. Non-limiting examples of such attempts are set forth below.
U.S. Pat. No. 4,229,396 to Suh, et al. teaches extruded synthetic resinous foams having an increased cell size when volatile fluid blowing agents are used by incorporating a cell-size enlarging agent into the extruded composition. The cell-size enlarging agent is an organic compound that is liquid at the foaming temperature and atmospheric pressure and generally soluble in the gel at the foaming temperature. In addition, Sub teaches that the cell-size enlarging agent melts below the foaming temperature of the foamable composition. Examples of cell-size enlarging agents include natural and synthetic waxes. It is asserted that foams prepared according to the disclosed method are more dimensionally stable than similar foams prepare without a cell-size enlarging agent.
U.S. Pat. No. 5,489,407 to Suh et al. describes a process for making a closed-cell, alkenyl aromatic polymer foam that has an enlarged cell size. The polymer is preferably polystyrene. For environmental reasons, the blowing agent is preferably an inorganic blowing agents such as CO2, nitrogen, argon, water, helium, or air. The composition used to form the foam includes a substantially non-waxy cell size enlarging agent. The composition may optionally contain a nucleating agent (e.g., inorganic substances such as talc, clay, and/or calcium carbonate) to control the size of the foam cells.
U.S. Pat. No. 5,475,035 and EP 0887167 to Park disclose processes that incorporate a first blowing agent into the polymer melt at a higher temperature and water as a second blowing agent at a lower temperature. The first blowing agent is substantially free of water, and includes inorganic blowing agents, organic blowing agents, and chemical blowing agents. The water is introduced downstream of the first blowing agent. Park teaches that by incorporating the water downstream of the first blowing agent, corrosion in the process equipment is minimized. In addition, a nucleating agent (e.g., inorganic substances such as talc, clay, and/or calcium carbonate) may be added to the melt to control the cell size of the foamed material.
U.S. Patent Publication No. 2008/0293839 to Stobby teaches the use of water and, preferably, at least one blowing agent selected from hydrocarbons, hydrofluorocarbons, and fluorocarbons as the blowing agent. The blowing agent may be incorporated or mixed into the polymer melt at an elevated pressure sufficient to prevent substantial expansion of the melt polymer material and to generally disperse the blowing agent in the polymer melt. It is asserted that the use of water allows for lower process pressures, lower foam density, and larger cell sizes while remaining environmentally friendly.
One problem with the use of water as a blowing agent is that it can cause corrosion in the process equipment. This problem is exacerbated when a brominated aliphatic fire retardant is utilized. For instance, the water may combine with hydrogen bromide (HBr) or other decomposition products of the brominated aliphatic fire retardants to form a polymer melt that has a high acid content. In turn, the acid within the polymer melt corrodes the hardware in the extrusion equipment. Because of the corrosion to the extruder and associated extruding equipment, parts must be frequently repaired or replaced, resulting in increased manufacturing costs and increased downtime.
Despite previous attempts to increase the cell size while using water as a blowing agent and reduce corrosion, there remains a need in the art to achieve an extruded polymer foam that has an increased cell size when non-HCFC blowing agents are used, that maintains the positive physical properties of conventional extruded polystyrene foams, and that reduces or eliminates corrosion of the extruder.